Inversion of enantioselectivity in the hydrogenation of ketopantolactone on a Pt-β-ICN chiral catalyst

The enantioselective hydrogenation of ketopantolactone (KPL) both in AcOH and in toluene was studied on a Pt-alumina catalyst modified by α-isocinchonine (α-ICN) and β-isocinchonine (β-ICN). The effects of the modifier concentration, temperature, solvents, and mixtures of modifiers on the reaction rate and enantioselectivity were examined under mild experimental conditions (253–298 K, 1 bar H2 pressure, 0.001–0.1 mM/L modifier concentration). Formation of excess (S)-PL was observed in the presence of α-ICN in both solvents (image, 37%). In the enantioselective hydrogenation of KPL over a Pt-alumina-β-ICN chiral catalyst, the major enantiomer was (R)-PL (image, 60%) in toluene, whereas in AcOH, (S)-PL (ee, 5%) was formed. The (R) configuration was opposite to what was expected from the absolute configuration of the cinchonine backbone; namely, inversion of enantioselectivity occurred. The conformational rigidity of both chiral modifier and reactant provides new insight into the possible structure of intermediate complexes in the enantioselective hydrogenation of activated ketones. The proposed structure of the intermediate responsible for the inversion of enantioselectivity is an adsorbed 1:1 β-ICN-KPL surface complex [C1 pro(R) and C2 pro(R)] in which the β-ICN acts as a nucleophile and binds KPL.